This invention relates to the field of fluxes and coatings for electronic components and devices and, more particularly, to moisture-proof coatings with solder-through capability and fluxing action.
Conformal coatings for electronic components and assemblies are well known and include the waxes used for temporary coating before, the soldering process and, as permanent coatings after assembly, thermo-setting materials such as acrylics, polyurethane, epoxies, silicones, and styrene. There are a number of serious disadvantages to each of these known materials such as the detuning effect due to high dielectric constant, electromigration effects due to moisture absorption, non-solder-through characteristic and non-repairable characteristic. For example, in the case of silicon coating materials, the silicone actually forms a chemical bond with many materials such as the surface of a printed circuit board and cannot be completely removed. Most other coatings must be scraped off thoroughly if replacement of a component is necessary, and then there is no simple way to replace the protective coating in the field. Also, there has been no way to coat components or assemblies for temporary storage before soldering without having to thoroughly remove the coating before the soldering process could proceed.
In soldering processes, whether hand soldering or mechanized; e.g. wave or flow soldering, a flux must be used first to prepare the surfaces to accept solder. While rosin fluxes have been widely used in electrical products to reduce corrosion at the soldered connections, it has also been the practice to remove all rosin remaining on the product after the soldering process. This additional step has been necessary because, while rosin flux is nearly inert when completely dry, residual activators may be sufficiently acid in the presence of slight humidity to allow the growth of metal "whiskers" (electromigration) between two adjacent points or areas having a voltage difference between them. Also, the moisture absorbed by rosin can, in certain circumstances, detune a circuit.
Another desirable coating application would be the capability for temporary non-contact, as of battery connections during a storage period, with a simple heating step allowing the contact to be made at the appropriate time, no contact cleaning being required before or after heating. Another application would be the elimination of the need for gold plating on contacts by the use of a coating which first protects from oxidation, then acts as a flux for the soldering process.
A solder flux has been developed for a specific "solder bucket" application which includes an oxidized homopolymer of polyethylene, suberic acid and fumed mica. The acid is suspended in the polyethylene and is the active fluxing agent. The mica acts as a thickening agent which is required in the particular application. Since oxidized polyethylene is somewhat hydrophilic, slight amounts of water can be absorbed, changing the dielectric constant significantly and thus affecting the tuning in some devices. Since the acid is not chemically bound to the polyethylene, it can migrate and cause high impedance shorts between electrical components, particularly when a substantial voltage differential exists.